When drilling for oil or gas, either in an exploratory well, or in a production well, it is often desirable to be aware of various physical characteristics of the geology surrounding a wellbore. For example, it is often helpful to be able to measure the temperature and/or pressure that exists at differing depths or distances along the wellbore.
A wellbore typically comprises of a hole drilled into the earth, into which is inserted a metal sectional tube, pipe, or casing etc., the resulting gap between the outside of the casing and the underlying geology (typically rock) being generally filled with a cement mixture which acts to restrict fluid movement and also acts as a bondline between the metal section tube and the surrounding geology.
One way of measuring the temperature and/or pressure within a well is to insert, into the gap between the metal casing and the rock (before the gap is infilled as explained above), a string having electrical sensors thereon that are connected with an electrical cable to the surface. This is relatively simple, but has significant disadvantages. Firstly, it requires the electrical connection running the length of the pipe (from the surface to the lowermost sensor), which can become broken, and so lead to disconnection of some or all of the sensors. Secondly, it is often undesirable to take this approach due to the potential influence that running a cable along the well bondline could have on the integrity of the well.